python: enable testing for 3.13

python: enable testing for 3.13

Python 3.13 is in beta and Fedora 41 is preparing to make it the default
system interpreter; enable testing for it.

(In the event problems develop prior to release, it should only impact
the check-python-tox job, which is not run by default and is allowed to
fail.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20240626232230.408004-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>